1. Field of the Invention
This invention relates generally to switching circuits and, more particularly, to a thyristor which is optically triggerable near the zero-crossing point.
2. Background Art
Triac drivers are designed for use with a triac in the interface of logic systems to equipment powered from 220 Vac lines, such as solid state relays, industrial controls, motors, solenoids, and consumer appliances. A triac driver is itself a triac, or thyristors connected in inverse parallel, and may be triggered by either a single gate terminal or by the application of light.
Triggering of the triac at high values of main terminal voltages is generally undesirable because of the generation of noise and the possibility of premature device failure because of high localized current density. Therefore, it is desirable to trigger the triac as near as possible at the zero crossing point of the AC main terminal voltage. Many schemes have been designed for achieving zero point switching utilizing thyristors. LEDs are generally used to provide isolation between the logical input and the main terminal voltages and load.
When a thyristor is comprised of a vertical transistor in conjunction with a lateral transistor, the triggering at high main terminal voltages has been inhibited by means of a monolithic MOS field effect transistor coupled between the gate and the cathode of the thyristor. However, the breakdown voltage is limited by the thickness of the gate oxide of the MOS field effect transistor. Since thicker oxide of the gate increases the threshold voltage, a higher breakdown voltage device will decrease the zero-crossing effect.
The gate of the MOS field effect transistor of this previously known circuit was connected to the base of the thyristor. The high voltage appearing at the gate of the field effect transistor and the inability to run the gate metal across a PN junction forced the use of an additional bonding pad for each thyristor.
Therefore, what is needed is a triac driver having an improved zero-crossing effect, a higher breakdown voltage, and fewer bonding pads than previously required.